Theoretical expectations for flux ratio between primary and offset secondary SDSS fibre in the case of a point source. Top panel: logarithmic light distribution of a point source assuming a circular 2D Moffat function with β = 3 and seeing of 2″. The circles indicate the two independent SDSS fibres with a diameter of 3″ where the primary is centred on the target and the secondary is offset by 25. Bottom panel: predictions for the flux ratios of the primary and secondary fibre for a point source as a function of secondary fibre distance to the point source. The curves are computed for three different seeing values of 1″ (black curves), 2″ (blue curves), and 3″ (red curves) with three different β values each as described in the legend. The measured [O III] λ5007 flux ratios for the six targets are shown as symbols; the error bar indicates the error on the combined fibre position uncertainty of 04 for the two fibres.